De novo designed copper coiled-coils as MRI contrast agents Giulia Molinaro, Melanie M. Britton, Anna F.A. Peacock University of Birmingham, UK This work reports on a new class of de novo designed Cu(II) coiled coils and their potential use as MRI (Magnetic Resonance Imaging) contrast agents. A Cu(II) coiled coil has been prepared which results in relaxivities comparable to that of the Gd(III) analogue, overcoming the perceived low relaxivity associated with Cu(II). With only one unpaired electron, Cu(II) has mostly been considered less promising than other paramagnetic metals within the development of MRI contrast agents. [1] However, the inclusion of metals within macromolecular structures such as de novo designed peptides, provides different strategies by which one can enhance relaxivity. Firstly, their longer rotational correlation time compared to small, fast tumbling molecules, is especially relevant at clinical/lower magnetic fields. Peptides may allow for the creation of extensive hydrogen bonding networks, as well as participating in proton exchange with bulk water. Two de novo designed peptides, MB1-2 and CS1- 2 are here tested in terms of their copper affinity and MRI relaxivity at both 1.4T and 7T. For Cu(MB1-2) 3 it is found that its relaxivities are comparable to Gd(MB1-2) 3 at both magnetic fields [2,3] (at 1.4T r 1 = 13.5 ± 0.5mM –1 s –1 and r 2 = 31.7 ± 0.5mM –1 s –1 for Cu(MB1-2) 3 compared to r 1 = 15.3 ± 0.6mM –1 s –1 and r 2 = 18.9 ± 0.1mM –1 s –1 for Gd(MB1-2) 3 ). Cu(II) binding to a series of coiled coils, with altered binding sites, is investigated. Conclusions are drawn between Cu(II) coordination chemistry, including water access, residence time and bond lengths, and the MRI relaxivity of resulting complexes. Peptide design can be used to control water access to the Cu(II) binding site, and in turn, tune the relaxivity of this class of potential contrast agents, in order to achieve high relaxivity complexes even with low-paramagnetic Cu(II). References 1. D. Schwert, J. A. Davies and N. Richardson, Contrast Agents I, 2002, 165-199. 2. R. Berwick, D. J. Lewis, A. W. Jones, R. A. Parslow, T. R. Dafforn, H. J. Cooper, J. Wilkie, Z. Pikramenou, M. M. Britton and A.F.A. Peacock, J. Am. Chem. Soc., 2014, 136, 1166-1169. 3. R. Berwick, L. N. Slope, C. F. Smith, S. M. King, S. L. Newton, R. B. Gillis, G. G. Adams, A. J. Rowe, S. E. Harding and M. M. Britton, Chem. Sci., 2016, 7, 2207-2216
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